Eye tracking is a technique for observing millisecond movements made by the eye, able to provide clues as to what people may be thinking. The primate visual system focuses its neurological resources around the fovea, a narrow spot (roughly 1.5° in extent) where photoreceptors are intensely concentrated. Because of this retinal geometry, people must move their eyes to perceive detail in objects that are of interest in their environment. People can make these movements voluntarily, or as an involuntarily response to unexpected events. For this reason, eye movements can be an indicator of attention, and therefore by examining these movements we can observe how attention varies among individuals under conditions relevant for multimedia learning.
Neurological techniques. The power of the eye tracking technique is limited primarily by the creativity and skill of the experimenter. A common and obvious application of this technique is to simply observe what people are looking at, to help quantify what it is that interests them. However, more advanced techniques can be used to investigate questions of neurological interest. For example, by manipulating the display in response to movements of the gaze, gaze contingent displays can investigate the effects of eye movements on visual perception, or to analyze complex questions, such as the rapid temporal dynamics of language processing involved in reading. Other techniques use low light levels, display motion, or color to isolate certain pathways and structures in the brain that are selectively sensitive to such stimuli. And when eye tracking is used with behavioral paradigms, eye tracking can be used to infer effects such as memory and attention.
Eye tracking parameters. Among the parameters most easily observed using eye tracking techniques are the following.
• Gaze Position — the angular coordinates of the monocular or binocular locus of the gaze.
• Fixation Duration – the amount of time the gaze is held fixed on object of interest; an indicator of cognitive processing.
• Fixation Rate – how often a person shifts the gaze during at task; indicates processing speed, attention, and memory.
• Pupil Change – involuntary response of the pupil to cognitive or emotional load.
• Eye Blink Rate – how often a person blinks provides information similar to pupil change.
Fixational motions. Even when the gaze is seemingly motionless, fixated on a spot, the eye continues to move, and this can provide additional clues to visual processing and perception. The most obvious movements are large, sudden (ballistic) movements (greater than 1 degree) called “saccades.” However, the eye also makes smaller such movements called “microsaccades,” thought to be indicative of shifts in attention, but also shown to act to correct gaze error, say to compensate for movements of the head and neck . These and other non-ballistic movements such as slow drift, and rapid tremors also constantly occur, and these may play a role in enhancing or maintaining visual perception.
Drawbacks. An important drawback of the eye tracking technique is that information about peripheral perception is not easily measured, and special behavioral methods must be used to infer what is being perceived and learned in the periphery. Another drawback is that the technique requires that the visual coordinate system be stable and accurately calibrated. This requirement often necessities use of a chin-rest to maintain spatial geometry in the experiment.